I think everybody is making a mountain out of a molehill here. Basically, all the wiring is already in place from the switch to the relay. The only change that is needed is the wiring at the switch itself: the center terminal on the switch needs to be changed from ground to +12V.
Simply find a convenient +12V line close to the switch location and tie in to it, using the appropriate fuse as recommended by the manufacturer. Depending on the location of the switch you might be able to run a line directly to your +12V distribution panel, or even right to the battery bank. No changes need to be made to any other wires.